Haloperoxidase active site

SCHEME 10.4 Structure and charge state around vanadium. 

X-ray studies have revealed that a major difference between chloroperoxidase and bromoperoxidase is found in one outer sphere histidine group in the bromop-eroxidase. In the chloroperoxidase, the histidine is replaced by a phenylalanine. The other two histidines are conserved. Proposals for the reaction sequence of the bromoperoxidase suggest that this histidine is involved in bromoperoxidase activity [46], Clearly, however, it is not necessary for such activity. 

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It is also of interest to point out that the amino acid sequence and structure of the active site of vanadium haloperoxidases is conserved within several families of phosphatases, with conservation of the amino acids involved in vanadate binding in one and phosphate binding in the other. 

Haloperoxidases are peroxidases capable of halogenating substrates in the presence of halide and hydrogen peroxide [14] or other reactions such as sulfoxidation, epoxidation and aromatic hydroxylation. Here, the halide ion is initially bound to the active site which may incorporate heme or vanadium or be metal free. The halide ion is incorporated into the substrate after electron transfer... 

Haloacid dehalogenase(s) 590 mechanism of 590 Haloalkane dehalogenase(s) 591 active site structure 591 Halocyanin 883 Haloperoxidases 855, 889 Hammerhead ribozyme 649, 651s mechanism of action 651 Hammett equation 308... 

Conte, V., O. Bortolini, M. Carraro, and S. Moro. 2000. Models for the active site of vanadium-dependent haloperoxidases Insight into the solution structure of peroxo-vanadium compounds. J. Inorg. Biochem. 80 41 -9. 

Many peroxovanadates have potent insulin-mimetic properties [1,2]. Apparently, this functionality derives from the ability of these compounds to rapidly oxidize the active site thiols found in the group of protein tyrosine phosphatases that are involved in regulating the insulin receptor function [3], The discovery of vanadium-dependent haloperoxidases in marine algae and terrestrial lichens provided an additional stimulus in research toward obtaining functional models of peroxidase activity, and there is great interest in duplicating the function of these enzymes (see Section 10.4.2). 

The haloperoxidases are a class of enzymes that catalyze the oxidation of halides via a reactive peroxometal active site. These enzymes are named according to the most electronegative halide they are able to oxidize. Hence, a bromoperoxidase can oxidize bromide and iodide but not chloride, whereas a chloroperoxidase can oxidize all three. Haloperoxidases are found in most living organisms and predominately fall into two classes the iron heme-based and vanadium-dependent enzymes. Of these, heme-based enzymes are found in mammals, where they provide a vital... 

Interestingly, there is a close structural correspondence between the active sites of the haloperoxidases and the acid phosphatases that allows both peroxidase and phosphatase activity from the two types of enzymes [49-51], For instance, recombinant acid phosphatases from both Shigella flexneri and Salmonella enterica ser. typhimurium, when substituted by vanadate, are able to oxidize bromide when in the presence of hydrogen peroxide. However, the turnover rate is quite slow, which is in accord with the phosphatase active sites not being optimized for peroxidase activity [52],... 

Fig. 5.18 Schematic representation of the mechanism of haloperoxidases. In the presence of Cl", HOCI is formed that (a) diffuses from the active site and oxidizes substrates in the medium, although in some cases, (b) oxidation may occur within the active site. In the absence of Cl", thiol-ligated haloperoxidases can (c) catalyze oxygen transfer to their substrates in a cytochrome P450-like reaction...

The X-ray structures of vanadium bromoperoxidases from the red seaweeds Corallina pilulifera and C. officinalis have also been determined and their structures are almost identical. The native structure of these enzymes is dodecameric and the structure is made up of 6 homo-dimers. The secondary stmcture of the chloroperoxidase from the ftmgus Curvularia inaequalis that will be discussed later can be superimposed with the Corallina hromoperoxidase dimer. Many of the a helices of each chloroperoxidase domain are structurally equivalent to the a helices in the Corallina hromoperoxidase dimer. This is in line with the evolutionary relationship between the haloperoxidases that will be discussed later. The disulfide bridges in the enzyme from A. nodosum are not found in the enzyme from Corallina and the two remaining cysteine residues are not involved in disulfide bonds. Additionally, in this enzyme binding sites are present for divalent cations that seem to be necessary to maintain the stmcture of the active site cleft. All the residues directly involved in the binding of vanadate are conserved in the algal bromoperoxidases. ... 

The overall amino acid homology between the three types of haloperoxidases is comparatively low there is 33% identity between the A. nodosum and Cor. officinalis enzymes and 21.5% identity between the A. nodosum and the Cur. inaequalis enzymes. There is, however, close homology in the active site regions. In the algal bromoper-oxidases, the active site is situated at the bottom of a substrate cleft or funnel, 20 A deep and 14 A wide in the case of Cor. officinalis, and 15 A deep and 12 (entrance) to 8 A (bottom) wide in the case of A. nodosum. The inside of the funnel is lined by hydrophilic and hydrophobic amino acids, allowing, in principal, access of a broad variety of substrates. 

VI-IX are structurally characterised, vanadate-inhibited phosphatases. VI, Rat prostat acid phosphatase VII, bovine phosphotyrosyl phosphatase VIII, mammalian protein tyrosine phosphatase PTP-IB (mutant Cys215Ser) IX, E. coli alkaline phosphatase. For comparison, the active centre of vanadate-dependent haloperoxidases (VHPO) (V), is also shown. The structures Xa and Xb have been proposed, based on EPR, for the vanadyl complexes formed with the PTP-IB active site peptide Val-His-Cys-Ser-Ala-Gly. 

Enzymatic halogenation catalyzed by haloperoxidases and perhydrolases involves the oxidation of halide ions to a halonium ion species which leads to the formation of hypohalous acids (Fig. 16.9-1). The products obtained by enzymatic halogenation with these enzymes are the same as the products obtained by chemical electrophilic halogenation with hypohalous acids. The differences in the para ortho ratios in the halogenation of some aromatic compounds could be due to a mixture of halogenation at or near the active site and in solution. 

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